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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="howto-interface-implement"></a>How To define implement an Interface?</h2></div></div></div>
<p>
      Once the interface is defined, implementing it is rather trivial. Source code showing how to do this
      for the <span class="type">IBaz</span> interface defined in the previous section is located in 
      <code class="filename">sample/interface/maman-baz.{h|c}</code>.
    </p>
<p>
      The first step is to define a normal GType. Here, we have decided to use a GType which derives from
      GObject. Its name is <span class="type">MamanBaz</span>:
</p>
<pre class="programlisting">
#ifndef MAMAN_BAZ_H
#define MAMAN_BAZ_H

#include &lt;glib-object.h&gt;

#define MAMAN_TYPE_BAZ             (maman_baz_get_type ())
#define MAMAN_BAZ(obj)             (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAZ, Mamanbaz))
#define MAMAN_BAZ_CLASS(vtable)    (G_TYPE_CHECK_CLASS_CAST ((vtable), MAMAN_TYPE_BAZ, MamanbazClass))
#define MAMAN_IS_BAZ(obj)          (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAZ))
#define MAMAN_IS_BAZ_CLASS(vtable) (G_TYPE_CHECK_CLASS_TYPE ((vtable), MAMAN_TYPE_BAZ))
#define MAMAN_BAZ_GET_CLASS(inst)  (G_TYPE_INSTANCE_GET_CLASS ((inst), MAMAN_TYPE_BAZ, MamanbazClass))


typedef struct _MamanBaz MamanBaz;
typedef struct _MamanBazClass MamanBazClass;

struct _MamanBaz {
  GObject parent;
  int instance_member;
};

struct _MamanBazClass {
  GObjectClass parent;
};

GType maman_baz_get_type (void);


#endif //MAMAN_BAZ_H
</pre>
<p>
      There is clearly nothing specifically weird or scary about this header: it does not define any weird API
      or derives from a weird type.
    </p>
<p>
      The second step is to implement <code class="function">maman_baz_get_type</code>:
</p>
<pre class="programlisting">
GType 
maman_baz_get_type (void)
{
  static GType type = 0;
  if (type == 0) {
    static const GTypeInfo info = {
      sizeof (MamanBazClass),
      NULL,   /* base_init */
      NULL,   /* base_finalize */
      NULL,   /* class_init */
      NULL,   /* class_finalize */
      NULL,   /* class_data */
      sizeof (MamanBaz),
      0,      /* n_preallocs */
      baz_instance_init    /* instance_init */
    };
    static const GInterfaceInfo ibaz_info = {
      (GInterfaceInitFunc) baz_interface_init,    /* interface_init */
      NULL,               /* interface_finalize */
      NULL                /* interface_data */
    };
    type = g_type_register_static (G_TYPE_OBJECT,
                                   "MamanBazType",
                                   &amp;info, 0);
    g_type_add_interface_static (type,
                                 MAMAN_TYPE_IBAZ,
                                 &amp;ibaz_info);
  }
  return type;
}
</pre>
<p>
      This function is very much like all the similar functions we looked at previously. The only interface-specific
      code present here is the call to <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-add-interface-static">g_type_add_interface_static</a></code> which is used to inform
      the type system that this just-registered <span class="type"><a class="link" href="gobject-Type-Information.html#GType">GType</a></span> also implements the interface 
      <code class="function">MAMAN_TYPE_IBAZ</code>.
    </p>
<p>
      <code class="function">baz_interface_init</code>, the interface initialization function, is also pretty simple:
</p>
<pre class="programlisting">
static void baz_do_action (MamanBaz *self)
{
  g_print ("Baz implementation of IBaz interface Action: 0x%x.\n", self-&gt;instance_member);
}
static void
baz_interface_init (gpointer   g_iface,
                    gpointer   iface_data)
{
  MamanIbazInteface *iface = (MamanIbazInteface *)g_iface;
  iface-&gt;do_action = (void (*) (MamanIbaz *self))baz_do_action;
}
static void
baz_instance_init (GTypeInstance   *instance,
                   gpointer         g_class)
{
  MamanBaz *self = MAMAN_BAZ(instance);
  self-&gt;instance_member = 0xdeadbeaf;
}
</pre>
<p>
      <code class="function">baz_interface_init</code> merely initializes the interface methods to the implementations
      defined by <span class="type">MamanBaz</span>: <code class="function">maman_baz_do_action</code> does nothing very useful 
      but it could :)
    </p>
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